Probiotiques et régulation de la réponse immune allergique et inflammatoire

Abstract

The intestinal tract contains a complex and dynamic ecosystem, composed of several hundreds of bacterial species, comprising potential pathogens, commensals or bacteria beneficial to the health of the host. Today, the important role of gut ecophysiology in human health is well established. The microflora established shortly after birth is essential in priming the immune system and contributing to gut homeostasis and intestinal function. The recent increase in diseases such as allergy and chronic intestinal inflammation (mostly observed in industrialized countries) has been attributed to our modern lifestyle, resulting from certain dietary, hygienic and medical habits. In fact these habits are thought to influence the normal development and later homeostasis of the intestinal microbiota, possibly also resulting in dysfunctional immune responses (it has been shown recently that recognition of commensal bacteria by toll-like receptors (TLR) at the host cell surface, plays a crucial role in maintaining homeostasis. Notably, the commensal flora could have an anti-inflammatory effect, through an inhibition of the nuclear transcription factor NFκB). Although there are more and more reports concerning the protective efficacy of probiotics, little is known about their precise mechanisms of action. Probiotic bacteria could exert beneficial effects through various mechanisms (e.g. competition with pathogenic bacteria, bacteriocin production, acidification, production of vitamins, production of beneficial enzymes), but research is now mainly focused on their immunomodulation properties. Lactobacilli and bifidobacteria can thus mediate different gut mucosal immune responses in a strain-specific manner (these responses clearly result from interaction and signalling at the local level but protective effects could also result through the liberation of soluble mediators. These signals could reduce intestinal permeability (and enhance barrier integrity), or act on the NFκB-mediated inflammatory cascade). Interactions with DCs (dendritic cells) and PRR (pattern recognition receptors) in the gut are believed to be involved in this communication and could lead to the development of regulatory immune responses. (It is obvious that development of dietary supplements containing strains beneficial to human health (substantiated by proper clinical trials !), would constitute a safe, efficient and cheap strategy to assist in the treatment or prevention of diseases which are linked to a dysbalance of the mucosal immune system). The clarification of the mechanisms by which bacteria or their metabolites reinforce intestinal homeostasis will allow a more efficient selection of active strains and will also help to determine optimal administration protocols. (These objectives represent a high priority in the field of probiotic applications.).